On the OM

One of the basic premises of Raja Yoga is that our perception of the divine Self is obscured by the disturbances of the mind. If the mind can be made still and pure, the Self will automatically, instantaneously, shine forth. Says the Bhagavad Gita: "When, through the practice of yoga, the mind ceases its restless movements, and becomes still, the aspirant realizes the Atman.” Perhaps a more familiar translation in the West, “Be still, and know I am God.”
ON THE OM
Chandogya Upanishad:
"One should meditate on this Syllable [Om]."2 If we do so, we shall certainly come to omniscience, for the upanishad also says regarding Om: "That is the quintessence of the essences, the Supreme, the highest."3 United with Om we are united with ALL. Om, then is not just the seed of omniscience, but of omnipresence as well. This union does not just produce a theoretical or intellectual result, but rather the mastery of life itself, for a little further on the upanishads says: "Speech [vak] and life force [prana] are joined together in the Syllable Om. Verily, whenever the pair come together, they fulfil each other's desire. He who knowing this thus, meditates on the Syllable, becomes, verily, a fulfiller of desires."
A little later the upanishad returns to the power of Om to fulfil desires, saying: "He obtains wishes by singing [intoning], who knowing this, meditates on the udgitha8 [Om] as the syllable. This, with regard to the self."9 The wielders of Om, being made wise, desire only the Self. And Om reveals the Self to them.
There may be various ways to approach the Goal, but Om is the Goal. Logically, then, the upanishad concludes: "This [Om] is the best means [of attainment and realization]; this means is the Higher and Lesser Brahman. Meditating on Om, one becomes worthy of worship in the world of Brahman."20 Om is that which transforms us, elevating our consciousness to the realm of the Divine and establishing it therein.
Mandukya Upanishad
"Om: this Syllable is all this. All that is past, the present and the future, all this is only the Syllable Om. And whatever else there is beyond the threefold time, that too is only the Syllable Om."21 From the original Sound, Om, all things have come into manifestation as Its extension-embodiments. Everything that has ever existed, now exists, or shall exist, is the expansion of Om. Om is all-embracing Eternity, containing and transcending past, present, and future. There is nothing but Om. That being true, the upanishad then says: "The Self [atman] is of the nature of the Syllable Om....Thus the Syllable Om is the very Self. He who knows It thus enters the Self [Supreme Spirit] with his self [individual spirit]."22 By means of Om, the eternal wave merges into the eternal Sea.
The Mundaka Upanishad speaks further on meditation.
"Taking as the bow the great weapon of the Upanishads [Om], one should place in It the arrow sharpened by meditation. Drawing It with a mind engaged in the contemplation of That [Brahman], O beloved, know that Imperishable Brahman as the target."23 The power of Om is emphasized by calling it a great weapon. Its intent and effect are serious and mighty-nothing less than union with the Absolute. It is called "the great weapon of the Upanishads" to indicate that Om, and Om alone, is the effective means recommended by the scriptures of Eternal Dharma for the realization of God. The japa and meditation of Om impel the consciousness of the yogi toward the Goal-Brahman. Moreover, it is the meditation of Om that "sharpens" the consciousness and renders it capable of union with Brahman. "The Syllable Om is the bow: one's self, indeed, is the arrow. Brahman is spoken of as the target of that. It is to be hit without making a mistake. Thus one becomes united with it [Brahman] as the arrow becomes one with the target."24 It is Om that ensures we will unerringly ("without a mistake") reach the Goal.

The Holy Science, Swami Sri Yukteswar
Parambrahma causes creation, inert Nature (Prakriti), to emerge. From Aum (Pranava, the Word, the manifestation of the Omnipotent Force), come Kala, Time; Desa, Space; Anu, the Atom (the vibratory structure of creation.)

The Word, Amen (Aum), is the beginning of the Creation.

IN THE BEGINNING WAS THE WORD, AND THE WORD WAS WITH GOD, AND THE WORD WAS GOD. HE WAS WITH GOD IN THE BEGINNING. THROUGH HIM ALL THINGS WERE MADE; WITHOUT HIM NOTHING WAS MADE THAT HAS BEEN MADE. IN HIM WAS LIFE, AND THAT LIFE WAS THE LIGHT OF MEN. THE LIGHT SHINES IN THE DARKNESS, BUT THE DARKNESS HAS NOT UNDERSTOOD IT.”
–John 1:1-5